The present invention relates to an air-core reactor having a magnetic shield and used for instance in a substation.
FIG. 11 is a perspective view, partially taken away, showing the general configuration of an air-core reactor in the prior art as described in Japanese Patent Kokoku Publication No. S57-39524, for example. FIG. 12 is a plan sectional view of the reactor of FIG. 11. As illustrated, a coil 1 is surrounded by a magnetic shield 2 comprising a plurality of frame-shaped magnetic sheets made of silicon-steel or the like, laminated with each other. Connecting members 3 pass through a central opening 1a of coil 1 and are clamped at both ends 3a between sheets of the magnetic shield 2. The connecting members 3 comprises a non-magnetic material such as stainless steel, an insulating material or the like.
When an alternating electrical current is applied to the coil 1 of the reactor, electromagnetic forces act on the side walls of the magnetic shield 2 facing the central opening 1a of the coil 1, causing vibration and noises. The connecting members 3 strengthens the rigidity of magnetic shield 2, suppressing these vibrations.
The electromagnetic forces acting on the side walls of the magnetic shield 2 facing the central opening 1a of the coil 1 increases in proportion to the square of the magnetic flux density. In large reactors, which have a high magnetic flux density, the increase in electromagnetic force is so great that it may overcome the force of friction between the ends 3a of the connecting members 3 and the sheets of the magnetic shield 2. Widening the connecting members 3 in order to enlarge their area of contact with the magnetic shield 2 and increase the friction force is not possible since the size of the connecting members 3 is restricted to the width of the central opening 1a of the coil 1. Friction at the ends 3a of the connecting members which are clamped between sheets of the magnetic shield 2 is thus incapable of fully resisting the strong electromagnetic attraction that occurs with a large alternating electrical current, leading to increase in vibration.